1. Field of the Invention
The present is related to mixer in general, in particular to low flicker noise mixer.
2. Description of Related Art
Mixers are an important circuit for radio frequency (RF) transmitters and receivers. The function of a mixer is to perform a frequency translation. While the present invention is generally related to and applicable to a variety of RF transmitters and receivers. In a direct-conversion receiver, a mixer is used to convert an RF signal into a baseband signal by mixing the RF signal with a clock signal, which is usually referred to as a local oscillator (LO) signal. The frequency of the LO signal is nominally the same as the center frequency of the RF signal, and consequently the resultant baseband signal is centered at DC. In a twist known as a low-IF (intermediate frequency) receiver, which is also highly relevant to the present invention, the frequency of the LO signal is slightly different from the center frequency of the RF signal, and consequently the resultant baseband signal is centered at a low intermediate frequency.
There are numerous topologies to construct a mixer circuit. A topology of particular relevance to the present invention is generally known as “Gilbert-cell,” which was taught by Gilbert in U.S. Pat. No. 3,241,078. FIG. 1A depicts a prior art Gilbert-cell mixer 100A comprising: a trans-conductance section comprising a pair of NMOS (N-channel metal-oxide semiconductor) field-effect transistors M1 and M2 for receiving an input voltage signal pair VI+ and VI− and converting the voltage signal pair into a first current signal pair I1 and I2; a switching quad 120 comprising four NMOS field-effect transistors M3, M4, M5, and M6 for commutating the first current signal pair I1 and I2 into a second current signal pair I3 and I4 in a matter controlled by a pair of complementary LO (local oscillator) signals LO+ and LO−; and a load section 130A comprising a resistor pair R1 and R2 for converting the second current signal pair 13 and 14 into an output signal pair VO+ and VO−. Throughout this disclosure, VDD denotes a first supply voltage, and VSS denotes a second supply voltage that is usually a few volts lower than VDD. The principle of Gilbert-cell mixer 100A is well known to those of ordinary skill in the art and well described in numerous publications (e.g. in RF Microelectronics by Behzad Razavi, Prentice Hall PTR, 1998, page 185), and is thus not described in detail here. In an alternative embodiment, the sources of M1 and M2 are coupled to a current source (not shown in the figure) instead of being directly tied to VSS.
A drawback in embodiment 100A is that the two load resistors R1 and R2 usually occupy considerable voltage headroom and thus adversely limit the available voltage swing for the output signal.
In an alternative embodiment 100B depicted in FIG. 1B, a different load section 130B is used. Here, the load section 130B comprises a pair of VCCS (voltage-controlled current sources) CS1 and CS2 forming a pair of active loads for the second current signal pair 13 and 14. Embodiment 100B further comprises a CMFB (common-mode feedback) circuit 140 for receiving the output voltage signal pair VO+ and VO− and a common-mode reference voltage VCM and generates a feedback voltage signal VFB accordingly to control CS1 and CS2. The function of the CMFB circuit 140 is to generate the feedback voltage signal VFB to control CS1 and CS2 in a closed loop manner such as to force the mean value of VO+ and VO− to approach the reference voltage VCM. The principle of common-mode feedback and circuit implementation are well known to those of ordinary skills in the art and well described in numerous literatures (e.g. in Design of Analog CMOS Integrated Circuits by Behzad Razavi, McGraw-Hill, New York, 2001, pages 314-324). Embodiment 100B usually allows greater voltage swing for the output signal, compared to the case in FIG. 1A where resistors R1 and R2 are used. In the applications of interest, a current source is embodied using a transistor. In a typical case for the mixer in FIG. 1B, each of CS1 and CS2 is embodied by a PMOS (P-type MOS) field-effect transistor. A MOS field-effect transistor always generates a type of low-frequency noise known as flicker noise. Therefore, CS1 and CS2 will generate a flicker noise, which is directly imposed onto the output voltage signal pair VO+ and VO−. The flicker noise is highly undesirable, in particular for a direct-conversion receiver, since the output signal contains some low frequency components that will likely be corrupted by a low-frequency noise.
What is needed is a Gilbert-cell mixer that has a load section that does not occupy considerable voltage headroom yet does not contribute flicker noise to the output.